1. Field of the Invention:
This invention relates to the purification of phosphoric acid partial ester mixtures produced by reaction of fatty alcohols with phosphorus pentoxide.
2. Statement of Related Art:
Among the phosphoric acid esters produced worldwide, the mixtures of mono- and dialkyl derivatives obtained by reaction of phosphorus pentoxide with alcohols are predominant.
The reaction mechanism has not yet been definitively elucidated. Similarly, the summation formula currently postulated for phosphorus pentoxide, namely -P.sub.4 O.sub.10 -, is not yet certain.
T. Fay, G. P. Sheridan and D. R. Karsa, Kommittee Espanol dea Detergentes, XII. Jornado (1981), point out that the phosphorus pentoxide normally marketed contains large amounts of high-polymer material and, accordingly, should retain the formula P.sub.2 O.sub.5.
In the production of phosphoric acid esters, especially by reaction of fatty alcohols with phosphorus pentoxide, the products contain varying amounts-depending on the molar reaction ratio of unreacted alcohol which, in many cases, affects the use of the products and is therefore almost always undesirable. The removal of the free alcohol is only possible by complicated, timeconsuming measures.
K. Sasse in "Houben-Weyl", Vol. 12, "Organische Phosphorverbindungen (Organic Phosphorus Compounds)", Part 2, pages 156 et seq describes the formation of phosphoric acid partial esters by the action of alcohols on phosphorus pentoxide. The free phosphoric acid is normally separated from the reaction product via the barium salts or, where relatively long-chain alcohols are used, via the sodium salts. The phosphorylation of sugars with phosphorus pentoxide is carried out at elevated temperature in the presence of tertiary amines.
Direct separation of the free alcohol by distillation is complicated by the elimination of olefins from the esters. In the case of short-chain alcohols, olefin formation actually becomes the main reaction. Chemical Abstracts, 95, 22 1704 w describes the neutralization of aliphatic phosphoric acid esters with an aqueous alkanolamine solution and subsequent extraction of the nonionic material with a low-boiling paraffin.
Published German Application 33 25 337 describes a process for the production of phosphoric acid diesters in which a base is added to the partial ester mixture in a quantity of from 0.5 to 1.3 equivalents, based on the phosphorus atoms in the mixture. The phosphoric acid monoesters are converted into o-phosphoric acid and organic hydroxyl compounds by hydrolysis in aqueos solution and, after the reaction step, are removed by distillation in the case of low-boiling alcohols or by fractional crystallization in the case of relatively long-chain alcohols. Phosphoric acid diesters which may still contain free unreacted alcohol are obtained.